The attachment and growth of marine fouling organisms (e.g., barnacles, mytilus, ascidian, bryozoa, tube worms, sea lettuce, and green layer) to ships, marine structures, fishing nets and cooling seawater intake lines in power generating plants cause various problems such as reduced ship speed, increased fuel consumption, mass death of fish due to poor flowing of seawater caused by the clogging of net meshes, and corrosion of structural members. Various antifouling paints are in current use for controlling the attachment and growth of marine fouling organisms.
Antifouling paints of the so-called self-polishing type are known; they release the antifouling agent into seawater while the paint film gradually dissolves into the seawater so as to continuously provide an antifouling property on the film surface. One great advantage of using antifouling paints of the self-polishing type as ship bottom paints is that even if they are unevenly applied to ship bottoms, the roughness of the coating surface is gradually decreased in the seawater, thus reducing ship-drag. Repair coating of a vessel at each dry-docking result in a thicker residual paint film, and this also causes increased surface roughness of the coating; this problem can also be avoided by antifouling paints of the self-polishing type, and any possible increase in ship-drag is prevented.
Prior art antifouling paints of the self-polishing type are described in Japanese Patent Publication Nos. 21426/1965 and 9579/1969. These paints contain a homopolymer of an organotin compound of the formula ##STR3## or a copolymer of such compound with another unsaturated monomer. These polymers are gradually hydrolyzed in weakly alkaline seawater to release a free trialkyl tin compound while carboxyl groups are formed in the polymer. The released trialkyl tin compound works as an antifouling agent to control biofouling.
The carboxyl group in the polymer renders it hydrophilic, and as more carboxyl groups are formed in the polymer due to hydrolysis, the polymer gradually dissolves into the seawater. By this mechanism, antifouling paint of the self-polishing type continuously provides an active exposed surface on the coating, thereby ensuring consistent slow release of the toxin throughout its service life.
The antifouling effect of the free trialkyl tin compound can be supplemented by an additional antifouling agent such as cuprous oxide. Furthermore, the self-dissolving action of the paint film allows the additional antifouling agent to be released at a substantially constant rate, thereby ensuring consistent antifouling effects.
The self-polishing property of the paint film is determined by the amount of trialkyl tin ester in the polymer, and the necessary self-polishing effect is not obtained unless said ester is present in a fairly large amount. However, organotin compounds having unsaturated groups are quite expensive, and using such compounds in large quantities is not cost effective.
Attempts have been made to introduce free carboxyl groups or hydroxyl groups into the polymer in order to impart self-polishing properties to the paint film without using a large amount of the trialkyl tin ester. However, these hydroxyl groups easily react with metallic antifouling agents such as cuprous oxide at ordinary temperatures, and the resulting cross-linking reaction, occurring in the paint container, produces a gelled paint which is no longer usable. Attempts have also been made to produce polymers with enhanced self-polishing properties by replacing part of the comonomers with relatively hydrophilic monomers such as methyl methacrylate, methyl acrylate and ethyl acrylate However, no satisfactory results have been obtained from such attempts.